Field of the Invention
The invention relates to an optical alignment liquid crystal alignment agent, a liquid crystal alignment film, and a liquid crystal display element. More particularly, the invention relates to a liquid crystal alignment agent capable of forming a liquid crystal alignment film having good ultraviolet reliability, a liquid crystal alignment film formed by the liquid crystal alignment agent, and a liquid crystal display element having the liquid crystal alignment film.
Description of Related Art
The liquid crystal display is widely applied in, for instance, television and various monitors. Among LCD display elements, the following are known liquid crystal cell LCD display elements: twisted nematic (TN)-type, super-twisted nematic (STN)-type, in-plane switching (IPS)-type, and modified IPS-type electrode structures . . . etc., and a fringe field switching (FFS)-type electrode structure . . . etc. increasing brightness by increasing the aperture ratio of the display element component.
The following is a known method for aligning the liquid crystal of liquid crystal cells: an organic film such as a liquid crystal alignment film is formed on the surface of a substrate, and a cloth material such as rayon is used to rub the surface of the organic film in a certain direction; silicon oxide is deposited on the surface of the substrate diagonally via vapor deposition; and a Langmuir-Blodgett (LB) method is used to form a monomolecular film having a long-chain alkyl group. In particular, from the viewpoint of substrate size, uniformity of liquid crystal alignment, treatment time, and treatment cost, a rubbing treatment is most commonly used.
However, if a rubbing treatment is used to perform liquid crystal alignment, then dust may be adhered to the surface of the alignment film due to dust or static electricity generated during the process, thus causing poor display. In particular, for a substrate having a thin film transistor (TFT) element, the generated static electricity causes damage to the circuit of the TFT element, thus causing reduced yield. Moreover, for the liquid crystal display element becoming more and more highly delicate in the future, with the high densification of the pixels, the surface of the substrate becomes uneven, and therefore it is difficult to perform a uniform rubbing treatment.
As a result, to avoid such undesired situation, a photoalignment method (such as Japanese Patent Laid-Open 2005-037654) provides liquid crystal alignment capability by irradiating polarized or non-polarized radiation on a photosensitive thin film. The patent literature provides a repeating unit having conjugated enone and a liquid crystal alignment agent having an imide structure. Accordingly, static electricity and dust are not generated, and therefore uniform liquid crystal alignment can be achieved. Moreover, in comparison to the rubbing treatment, the method can precisely control the direction of liquid crystal alignment in any direction. Furthermore, by using, for instance, a photomask when radiation is irradiated, a plurality of regions having different directions of liquid crystal alignment can be formed on one substrate in any manner.
However, the liquid crystal alignment film has the issue of poor ultraviolet reliability. Specifically, after the liquid crystal alignment film is irradiated by ultraviolet for a period of time, the situation of significantly reduced voltage holding ratio of the liquid crystal display occurs, thereby causing issues such as reduced contrast of the liquid crystal display.
Therefore, how to provide a liquid crystal alignment agent capable of forming a liquid crystal alignment film having good ultraviolet reliability such that high voltage holding ratio is still maintained after prolonged ultraviolet irradiation when the liquid crystal alignment film formed thereby is used in a liquid crystal display element is a current issue those skilled in the art urgently need to solve.